Virtual Vault going missing

We have deployed Virtual Vault to our Enterprise Vault v9.0.2 environment for our 10,000 users, however we are getting approximately 50 calls a week to our helpdesk stating that Virtual Vault has gone missing from Outlook. We then have to jump onto the users machine and tick the box next to the users name on the Virtual Vault tab and this restores the users access.

So, my questions :)

1) Has anyone come across an issue like this before and if so how did you fix it?

2) Is there a way to force this tick box to be on by way of regkey/ev policy/gpo in order to prevent the user getting the issue?

No the VVEnabled isn't set on either the users that are working, or the ones that are, so I don't think thats the issue. Also none of our users are connecting via RPC/HTTP, they can only connect via MAPI.

I did notice that the HKCU\software\KVS\Enterprise Vault\*a long number*\archiveID\*AnotherLongNumber*\ DWORD changes from 0 to 1 when the Virtual Vault tick box is ticked, however just changing this value in the registry doesnt restore VV access to the users

'Make Virtual Vault availible to users' is ticked in the policy, however when I go the profile of both users who can see VV in Outlook, and those who can't see it, VVEnabled is not present in the registry. Adding this to the users who cannot see their VV does not enable the VV view when they restart Outlook.

Any updates on this? I had worked on similar case where we had the following scenario.

During migration of Exch2k3 to Exch2k10 the quest CPUU(Client Profile Updating Utility) makes various changes to the client’s files so outlook points to the new servers.
Post this once the OST is rebuilded the Virtual disappears and needs to manually select the check box to bring it back in Outlook.